Would a Patient with Iron Deficiency Anemia Have Microcytosis?
Yes, a patient with iron deficiency anemia would be highly likely to have microcytosis, as reduced mean cell volume (MCV) is a classic and characteristic finding of iron deficiency. 1
Mechanism and Characteristic Findings
Iron deficiency impairs hemoglobin synthesis, which leads to the production of smaller red blood cells with reduced hemoglobin content. 2 The key laboratory findings include:
- Microcytosis (reduced MCV): Defined as MCV <80 fL in adults, this is a hallmark feature of iron deficiency anemia 3, 4
- Hypochromia (reduced MCH): Mean cell hemoglobin is probably a more reliable marker than MCV, as it is less dependent on storage conditions and the counting machine used 1
- Both parameters lose sensitivity in the presence of chronic disease, thalassemia, or vitamin B12/folate deficiency 1
Important Clinical Nuances
Mean cell hemoglobin (MCH) may be more sensitive for detecting iron deficiency than MCV alone, and a reduction occurs in both absolute and functional iron deficiency. 1 This is particularly important because:
- Microcytosis can be masked by concurrent vitamin B12 or folate deficiency, which causes macrocytosis 5
- Elevated red cell distribution width (RDW) can indicate combined deficiency states (iron plus folate/B12) 5
- In combined deficiencies, the MCV may appear normal while MCH remains reduced 1
Differential Diagnosis Considerations
While microcytosis strongly suggests iron deficiency, specificity is limited because microcytosis also occurs in: 1
- Thalassemias: MCV is typically reduced out of proportion to the level of anemia 1
- Sideroblastic anemia: Genetic disorders of heme synthesis 1
- Anemia of chronic disease: Though the MCV rarely falls below 70 fL (lowest reported is 61 fL) 6
- Hemoglobinopathies: Such as homozygous HbE disease 2, 7
Diagnostic Approach
To prevent unnecessary investigation, hemoglobin electrophoresis is recommended in patients with microcytosis and normal iron studies, particularly if there is an appropriate ethnic background suggesting thalassemia. 1
The diagnostic workup should include:
- Serum ferritin: Most specific test for iron deficiency; <15 μg/L is highly specific (specificity 0.99), while <45 μg/L provides specificity of 0.92 1
- Transferrin saturation: Severely reduced (e.g., 3%) in iron deficiency 5
- Serum iron and total iron-binding capacity: Classic pattern shows low iron with elevated TIBC 2, 3
Common Pitfall
Do not assume all microcytosis is due to iron deficiency. If iron studies are normal or show elevated ferritin with microcytosis, consider thalassemia trait (elevated hemoglobin A2 >3.5%), genetic disorders of iron metabolism (elevated ferritin and/or transferrin saturation), or anemia of chronic disease (low iron with decreased TIBC). 1, 3